When did Pennsylvania remove the "flintlock only" restriction?
Short answer they didn't, the regulations that I posted are for general firearms season. They have in the past few years changed Flintlock regs to allow different sights, projectiles, and crossbows. Pennsylvania is strange in that you can use a crossbow in muzzleloader season but NOT archery season, except for a few exceptions.
Long answer the added an early ML season in which you can use any muzzleloader.
Muzzleloader Seasons: October Antlerless Deer Season - Any
single-barrel muzzleloading long gun with flintlock, in-line or percussion
ignition 44 caliber or larger, or 50 caliber or larger handgun.
Scope sights permitted.
Late Flintlock Season - Flintlock ignition, single-barrel long guns
manufactured prior to 1800, or a similar reproduction of an original
muzzleloading single-barrel long gun 44 caliber or larger, or 50 caliber
or larger handgun, using a single projectile. It is unlawful to use
telescopic sights. Crossbows are permitted during the muzzleloader
seasons. Hunters using crossbows during the early muzzleloader
season or late flintlock season would need to have a muzzleloader
stamp in addition to their general hunting license and appropriate
WMU antlerless deer license. However, late season flintlock hunters
using a crossbow would be permitted to take an antlered deer or
an antlerless deer anywhere in the state with their unused antlered
deer tag, just like other late season flintlock hunters.