I discussed this a bit on the Unity Discord already but I imagine there are two things going on here right now:
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Unity wants to move more packages to Unity core, basically ship only select version for that engine version by default and if you want to change it, you have to change the manifest manually (either via text editor or use “add git package” -route from PM and just put the name of the package there). More about packages as part of Unity core here: What is next for us at Unity with Scriptable Render Pipelines .
afaik this also isn’t new to 2021.1 but also can be seen on latest 2020.2 betas already as it ships with more packages included by default. -
Current alpha hasn’t really gotten updated packages yet since it’s still so early. For SRPs and 2021.1, we’d expect to use 11.x packages which aren’t available yet. in fact Unity only yesterday bumped the master branch on Graphics repo to 11 (Bump version to 11.0.0 by sebastienlagarde · Pull Request #2294 · Unity-Technologies/Graphics · GitHub)
Of course for the time being starting with 10.x series rather than 8.x on 2021.1 would be better regardless