(2019). What's the latest on sub-$100 FPGAs this decade, anything significantly better than the Mojo I picked up back in 2013 to tinker with?

Depends on what you're wanting to do.

The OSS toolchains have matured a lot since then, with parts from Lattice being fairly well supported if you don't want to touch a vendor's tooling.

Gowin have some of the more interesting new parts which sit on a different part of the cost/feature curve. The Tang Nano boards seem fairly popular but I only have experience with far larger GW5 parts.

The newer parts from Intel/AMD have increasingly impressive hard-peripherals and internal features, but not that relevant unless you know what you want/need.