User’s scripts will anyhow need to be modified.
The old ROOT versions do not provide “--python2-version” nor “--python3-version”.
If “--python-version” returns both versions (ordered, first any 2.x then any 3.x) then the required modifications would be trivial, try e.g.:
P="2.7.18 3.8.3"; echo "${P%% *}"; echo "${P##* }"
If a user is just interested in python2, instead of “${P}” one just needs to use “${P%% *}”.
If a user is just interested in python3, instead of “${P}” one just needs to use “${P##* }”.
This trivial modification will make the script work in any ROOT version, also in any old one (in which “--python-version” returns just one python version, if any).