[2021] Will Alpha 20 require a new save file?

Originally posted by Lord-Knight Fandragon: Hol up, we have to restart our whole game when they update the game?

Not for every update. Just for Major updates.
EG: You do not need to restart for A19.0->A19.1, Or A19.1->A19.2; These are 'Minor' updates that, for the most part, your save should be forward compatible with.

However, For 'Major' updates like A16.x->A17.x, or A17.x->A18.x, the saves are not compatible. The time span between updates is significant though. For a minor update it might be a month or so, But for a new Major update to come along, it can be as much as a year; More than enough for you to happily start and finish a save.

And as mentioned, you can always opt into a specific version to continue your save; You just won't be able to play with stuff from the newer update(s) in that save.