Even in the very latest snapd version, your only choices are;
- Pause all updates, but only for 60 days max
- Revert a single snap, but only until author releases a new version. This can be only done once (twice on some systems), so if you want even older versions, you are out of luck.
This policy seems very inappropriate for Linux systems where people traditionally value having a control over the code they run. It is much closer to Android/iPhone "app store" model -- but even those at allow disabling automated updates!
[0] https://forum.snapcraft.io/t/disabling-automatic-refresh-for...