Old beetle holes can last for decades
Beetle holes do not disappear when the insects have gone. It is quite normal to see them in old roof timbers or floorboards, sometimes long after the infestation ended.
The amount of damage varies
A few surface holes in sapwood are very different from extensive galleries through a small joist or floorboard. Strength should be judged from the sound timber remaining, not from hole count alone.
Previous treatment can confuse the picture
Old roofs sometimes show drill holes, spray staining or treatment records from earlier decades. That history does not prove whether beetles were active then or are active now.
Avoid treatment simply for reassurance
Historic England specifically notes that treatment is not generally necessary for historic insect damage where the infestation is no longer live. Evidence of current activity should come first.
Useful technical sources
These links provide background guidance. A particular building still needs its own inspection.