The closest thing to Super XX avail at the moment in terms of curve shape and linearity is Fomapan 200; but it differs in many other respects.
In practical terms, however, TMY400 is probably the most suitable substitute, and has a lot to commend it in its own right. And the closest thing to Plus-X curve-wise is probably Delta 100, but not development wise. FP4+ is an excellent general-purpose film somewhere in between.
There is no shortage of superb black and white sheet films at the moment, though an old favorite or two have obviously disappeared.