From the list https://scikit-learn.org/stable/related_projects.html, I propose to remove

• svmlight-loader
  Reason: Not installable via pip install, not license
• Neptune because it is not really usable without an account. Was added in DOC Add Neptune link to "Experimentation frameworks" #20767.
• rep
  Reason: outdated (last commit in 2016), requires scikit-learn == 0.17.1
• nolearn
  Reason: outdated (last commit in 2019), requires scikit-learn==0.17
• Lasagne: outdated
• Kernel Regression
  Reason: outdated
• LibOPF
  Reason: not installable via pip install
• Spotlight
  Reason: not installable via pip install or conda install (only some non-standard channel)
• MSMBuilder
  Reason: github repo has been archived

Remarks:

• mlxtend is listed twice
• LightGBM should be listed

BTW: We should set some standard as to what we require to be listed.