In the first Guava release of 2020, we plan to remove support for sending Guava types over GWT-RPC.
We plan to continue to release guava-gwt (without GWT-RPC support) beyond then, though we may increasingly need community help, as our past GWT experts have moved on or stopped keeping up to date on GWT changes.
We're dropping GWT-RPC support because, over time, it has cost more to maintain, and it's become less used as other RPC systems gain popularity. In more detail:
GWT-RPC has been buggy, and we've had to spend time on workarounds. More generally, our GWT-RPC support has required significant maintenance, and it's delayed features like hash-flooding protection and Android optimizations.
We suspect -- but do not have good numbers on this! -- that few Guava users are using GWT in general, fewer are using GWT-RPC, and still fewer are sending Guava types over GWT-RPC.
For advice on alternatives to GWT-RPC, we suggest discussing on the GWT mailing list. For other concerns, let us know here on guava-discuss@.