Any FP code generated by the compiler is potentially impacted for the negative. If you use some of the bits under patented functionality, i.e. AV transcoding, I would anticipate a negative impact. But in that case you have to build it yourself and probably have gone to NEON anyways.
Do you have a reference
For wrtpac devices > mamba the change cut the FP registers available for use in half; assuming the compiler respects options provided. I have not bothered to go and look at the assembler churned out, but as the device that did not work now does...
OpenVPN would depend on SSL library used. I would expect no impact with openssl, I have not taken a close look at mbedtls so not sure.
Easy enough to load an image with / without on the two partitions and test whatever the bits are that are of concern to you. But the change to go back from whence we came is trivial, so I just do that and fuhgeddaboudit.