Right now we ship only FTS extension for SQL Server, but people keep asking for them for other databases:

• sqlite (we already have it in tests) https://github.com/linq2db/linq2db/blob/master/Tests/Linq/Linq/FunctionTests.cs#L471
• mysql https://stackoverflow.com/questions/54408134/how-to-make-full-text-search-with-linq2db

Looks like it makes sense to implement it as a part of linq2db as Sql.Extension is not something easy to implement by user due to lack of documentation/knowledge of linq2db internals