10 days ago - Shurenai - Direct link
There hasn't even been an update recently. The new update hasn't come yet, and the last update was like 6+ months ago. TFP did literally nothing to screw up your game in this case.

You trusted Steam, a program that automatically updates your games to keep your 'locked in version'. You should've made a copy of the directory outside of Steam's folder if you wanted it 'locked in'.

Set your beta to 20.7, and follow these steps except don't wipe your saves or world. You've probably just got some file conflicts going on.
https://steamcommunity.com/app/251570/discussions/1/3110269850346164483/#c3110269850345972067

Edit: It occurred to me that TFP hadn't yet removed the A20 individual versions; 20.0, 20.1, 20.2, 20.3, etc, and may have done so finally. They leave the additional minor versions of the preceding major version available for a time, but they always eventually get removed- With 1.0/A22 coming now, That means A21 is now the new preceding major version, and so the 20.x versions likely got removed leaving only 20.7, which was the last minor version of A20.

They've been doing this since Alpha 8. That's why it's only got, ie, A16.4, A17.4, A18.4, A19.6; The final minor versions of each major build.

A20.1, A20.2, and on up to A20.6 were never going to stay in the beta list forever.

Speaking to everyone who reads this but OP included, You really should, if you care about a specific version, make a backup of the entire game folder outside of steam's control, or if anything happens within or even outside of TFP's power and that version gets delisted (Perhaps Steam screws something up or decides to hard limit the number of available betas, for example), your game WILL update, Because that's what steam does- It automatically updates your games.

Anyway; Following the link above will likely get your game working again, OP.