[ Mod Request ] Stuffs that could train your townies.

[The Black Raven]

Seriously, I am tired having my colonist loses a skill point in melee just because raids doesn't come everyday...

[Canute]

Maybe this is the right for you ?
[B18] Static Quality Plus B18
- modify whether or not you can lose skill levels due to degradation
https://ludeon.com/forums/index.php?topic=24718.0

or

Misc.  Training

This adds some training facilities:
Shooting Target
   Train up your shooting ability. Joy and training possible!
Martial Arts Dummy
   Train your martial arts ability. Joy and training possible!
Archery Target
   Your colonists like to shoot some arrows for joy. **Joy only**

https://ludeon.com/forums/index.php?topic=3612.0

[The Black Raven]

Oh, Danke... Finally... Getting tired having my assassin go as low from 19 to 17.

[calantlar]

Another option is the MadSkills mod which can be used to disable the XP loss entirely or in a tiered fashion.