Yes, Belle Isle is still around, and it's still cash only.

Luke's Lobster isn't anything special, but it's also not bad. The quality of the lobster and crab they use is decent, which makes it mildly more expensive.

Alive and Kicking Lobsters in Cambridge still has their infamous lobster sandwich. Sullivan's Castle Island (they have a second location in the TD Garden food hall now) does a good small no-frills lobster roll as well. (Not sure what they charge these days, but I recall the price being no frills as well; I think it was $12.95 in 2019?)

Skip Kelly's Roast Beef for seafood. They have been on a consistent decline. Their airport locations (two locations in terminal B, and one in terminal C) aren't great either. They're serving a lobster roll that's lesser in quality than the no frills one at Sullivans but they're charging over $30 for it.