Apache Commons Math Optim: should I use 4.0 or 3.6.1? 4.0 seems dead, but 3.6.1 is deprecated…

Question

I want to use Apache Commons Math for Optimisation, but 3.6.1 contains APIs that have been deprecated in anticipation of 4.0, however, the latter seems to have received little development since 2016.

Any advice on which version to use would be greatly appreciated.

(As an aside, any advice on better open source unconstrained optimisation libraries for java would also be appreciated.)

thanks!