You'll have to add a custom channel for now but please give this a test.
Quick test and adding 2nd Custom channel appears to work in as expected, was then able to select 10.0 as an update channel and see latest 10.0 Nightlies. I did not attempt to update (or downgrade) yet, as am already on latest nightly. 
flubshi as mentioned by Smultie LibreELEC Nightlies now being identified with "10.0-nightly" within filenames since 27th Feb.
Note: Currently no "LEnightly-10.0" update channel to select
Would be great if you can update your temporary workaround to provide access to latest nightly builds 
v19.7.1 auto-magically installed on rpi4 running LibreELEC Matrix nightly builds (sometime in last few hours along with three other addons) and working great and as expected. 
Thanks HiassofT Attempted to manually install add-on, but kodi fails to install the zip.
LibreELEC Kodi.log with debug switched on.
Looking at LibreELEC Matrix Nightly addons HERE there does not appear to be an update from v19.6.1 to v19.7.1 for Rpi2 and Rpi4 unlike for other devices in listing.
I note following ARMv8 zip, is this compatible for use with Rpi4 as its not being selected from with LibreELEC when requesting repo to be updated with latest versions of available addons?
Thanks for blog update surrounding LE10 release plans and support level expectations for varied LibreELEC capable devices 
Just checked and pvr.zatto v18.1.23.1 update has been automatically applied to rpi3 (LibreELEV 9.2.6) working fine again
Upgrading from Leia to Matrix and expecting existing add-ons to be working after will be hit and miss. Personally I would try out LE10 as clean install on spare sd (then you can still switch between until your happy with Matrix)
9.8 is actually allocated to/for LE10 builds
All nightly MATRIX builds can be found HERE filter on rpi4 and select latest build etc
You don't say what S912 device you have but I understand that following is latest LibreELEC Matrix Nightly that I would consider installing on my Khadas Vim2 S912 device. (Not using at moment, too much noise from it small cooling fan, prefer stability of Rpi4)
LibreELEC-AMLGX.arm-9.80-nightly-20210212-bb390f9-box.img.gz
All nightliy MATRIX builds can be found HERE
Note: There isnt an offical build for Leia
Ref previous post above, this pvr.zattoo issue 114 with response from rbuehlma in GITHUB, appears to answer my query that update is on its way for Matrix too.
For overall info in this thread double checked on rpi3 (Leia) and rpi4 (Matrix) and installed Zattoo pvr Clients are currently v18.1.21.1 and v19.6.1.1 respectively, with LibreELEC addons repo currently offering same versions. Keeping an eye out for updates to check that pvr.zattoo is working correctly again once update appears in LibreELEC repo. Thanks to all involved in maintaining, updating and improving LibreELEC.
--edit-1--
Thought to quickly perform zattoo.pvr check in Kodi (Matrix) running in Win10 and confirm v19.7.1 installed and currently OK
--edit-2--
Noted: kodi-binary-addons: update to latest versions by HiassofT · Pull Request #5134 · LibreELEC/LibreELEC.tv · GitHub Thanks HiassofT
Whenpvr.zattoo fix/update becomes available in repo for LibreELEC 9.2.6 will it be available in repo for 9.8 Nightlies as well?
nightly-20210209-06bafb5 Kodi.Log with debug switched on, indicating login failure.
Current latest Dec 2020 6.9.3 pre-release build for Matrix, can be found in GitHub Here
Note: You will need to download and install add-on manually.
I have earlier Nov 2020 pre-release 6.9.1, which has been working fine for me on rpi4 with 9.80 nightly builds.
We got a fix for the resume issue, it should find it's way into nightly builds soon ffmpeg: update rpi patch by HiassofT · Pull Request #5093 · LibreELEC/LibreELEC.tv · GitHub
Ref LibreELEC-RPi4.arm-9.80-nightly-20210205-3420901 nightly.
Additional feedback that ffmpeg fix, appears to fix issues I have recently observed in previous nightlies with search through and playback of catch-up TV programs when using pvr.zattoo. Prior to this fix being introduced, initiating stream was generally slow to commence, although fine once started then slow and erratic to recover when attempting to FF/RR through a catch-up stream. (used mouse in order to selectively step fwd through stream as previous workaround)
Now interaction is clean and immediate. LibreELEC settings Kodi.Log
I have been sticking with and enjoying latest nightly Kodi Matrix Beta/LibreELEC
Alpha nightlies on my rpi4 for a while now, working extremely well for my daily
usage needs. Thanks to everyone involved in ongoing development, very much
appreciated.
Good to know, I upgraded from 20210116 to 20210117 earlier today on my rpi4; noting similar disablement of pvr.zattoo etc. before rolling back to previous build. Will try 20210117 nightly again later today.
Reference Chewitt's suggestions to you and your comment implying you’re intending to use LibreELEC with rpi3
Note: Installing LibreELEC to SD via Raspberry Pi Imager or LibreELEC USB-SD Creator should not make any difference?
(Personally never used Raspberry Pi Imager to install LibreELEC onto SD)
Raspberry Pi 4: https://up.legg.ga/rpi4/
Note: This uses the official LE nightly builds from Index of /
Thanks for sharing/providing temporary solution, quickly setup custom update URL in settings and manual updated to latest nightly for rpi4.
Working as expected. 
