Such episodes as entering each other's airspace happened tens (if not hundreds) of times between the US and Russia in the years of the "cold war".
The vast majority of those flights didn't (and still don't) involve
border penetrations.
They may routinely fly near or into Air Defense zones (formal or informal ADIZ) but actual, deliberate penetrations by US or Russian armed military aircraft into each other's sovereign borders are not the norm and would be considered highly provocative.