Priority Pass has not made any acceptance distinction between Select and regular memberships for a decade. Instead, they introduced the Lounge Club brand with a reduced network. I don't see Collinson reversing direction for Chase, Their lounges will either accept all Priority Pass memberships or none, BUT they could easily have a No Guests policy for Priority Pass entrants, and a different policy for Sapphire Reserve entrants. Guest policies are always set by the lounge, not by Priority Pass or the membership sponsor.
They have not had differences in lounge access for a while, but they still have the distinctions on the cards, including after the logo update a few years ago which would have been an easy time to remove it from new cards. The distinction was put in place because United had an agreement with Chase that precluded other cards from having access to their clubs. There has not been a reason to have lounges excluded from Select since then, but it seems like something that could happen again if Chase says the only people who can access with a credit card are those with specific Chase cards. The identifiers are already in place, and I'd expect their systems would still be setup to allow exclusions (especially given Amex PPs are not accepted at PP restaurants).
If Chase does somehow allow competitor cardmembers in (and that seems a bit of a stretch, I can't imagine execs would sign off on that), I fully expect there to be in-lounge benefits for Chase cardholders, such as a better food menu or the ability to order higher end alcohol or beverages. I say that as someone who does not have any Chase card that I would expect to give me access, but I cannot see them simply opening the gates for all PP members who got their cards through Amex/Citi/C1/etc.