It was state senator Boscola's (D) bill and ultimately signed by Wolf (D).. It was a bipartisan bill in which state R's were initially willing to accept the no excuse required absentee voting that the D's wanted in exchange for eliminating the party line vote, amongst other things. It wasn't an R bill and the expansion of mail in voting was something the D's were pushing, not the R's.

Granted, the R's did ultimately vote for it. And a good number of D's upset with the amendments ultimately chose to not vote for it. Ironic that it's now the R's who want it deemed unconstitutional and thrown out. They claim they were "duped" and the bill doesn't represent what they voted for... Mastriano has written a couple of op-eds on the subject.

Perhaps it's not an issue that falls along a strict partisan divide but you need to look beyond just PA to GOP opinions across the board. Jeff Essmann, the Montana GOP chair didn't hold back his thoughts when his state was debating this in 2017 (long before Trump weighed in on any of this). I lived in WA state for a while and all the pushback I saw to the mail in ballots were coming from R's... and this was back in 2008ish when I was stationed there. Here's Texas AG Ken Paxton's take on the topic granted this came after 2020's general election.

There's also the issue of GOP obsession with voter ID laws which don't line up well with any sort of mail-in ballot which dates well before the 2020 election and any input from Trump sycophants.

And of course there was also this slip up to the SCOTUS, in which a GOP lawyer admits that more restrictions benefit the GOP... reducing mail-in ballots is one form of restriction although maybe the PA GOP wasn't in on the game in 2019 when they ultimately voted for Act 77 before turning on it.