T-SQL90 vs Transact-Sql
I suspect that the 90 refers to the deprecated 90 compatibility level, which is SQL Server 2005.  It is most likely there to support legacy code, as important changes to T-SQL syntax were made in SQL Server 2008, causing previously acceptable code to fail unless the compatibility level was downgraded.
The most likely reason that you are unable to find out further information is that Microsoft have removed a lot of online documentation for versions of SQL Server that are no longer supported.
The reference below seems to be consistent with this use of &quot;TSQL90&quot;.
https://docs.microsoft.com/en-us/dotnet/api/microsoft.sqlserver.transactsql.scriptdom.tsql90parser?view=sql-dacfx-150