@ioncube I think you mean legacy 5.10 (not 5.1).
The code to build legacy 5.10 is still there (https://github.com/armbian/build/blob/b8dbe607e59a9eb23c70f129b60e2df2b08cdb01/config/sources/families/rk35xx.conf#L19-L27)
But as igor commented above:
So it is unmaintained and may no longer build.
To use the legacy branch you need to specify it on the command line: BRANCH=legacy