If a unit is destroyed in combat, is it permanently destroyed?

If a Mech is destroyed, it cannot be used in that combat temporarily but is not permanently destroyed.
However, if it is used for a certain period of time, it will need to be repaired, and on the Mech Info page, you can repair by consuming resources.

Also, in PvP, Ekarium can be consumed to redeploy eliminated Squads after a certain amount of time,
and if you move Squads with destroyed Mechs to your base, you can restore durability and redeploy destroyed Mechs.