Here's what I found so far

approach0.xyz : offline

searchonmath.com : Gives irrelevant results such as p = m v when searching for F = m a

https://search.mathweb.org/: A collection of abandoned projects and offline sites

in no particular order:

https://matlas.ai

https://www.mathdeck.org

https://portal.mardi4nfdi.de/wiki/Portal

https://www.theoremsearch.com/search

https://ansumandas441.github.io/mathematical-discovery-engin...