The issue is largely that the OpenBSD authors don’t really seem to understand how actual ROP chains work, so their mitigations are tailored towards things that don’t actually match reality. For example, trapsled is meant to protect against sloppy/partial overwrites writes that take advantage of nops to slide to where attackers actually want to go. The problem is that using nop slides in a ROP chain is Not A Thing. People don’t do it, it’s literally a thing they thought sounds like a reasonable attack but it actually protects against nothing in the real world. Similarly, gadget removal is cool and all but their understanding of how well it works is very flawed. Their analysis relies on really dumb, off-the-shelf gadget finders for one. But more critically, they claim something like 90% gadget reduction as if this actually protects against a significant majority of attacks. It does not. Exploit developers are usually not constrained by the gadgets they have available to them. If there are a thousand gadgets and you reduce it to hundred, yes you probably killed a lot of good gadgets…but in the remaining ones there’s usually enough to still perform whatever you want, maybe with a little more difficulty.
These mitigations don’t really harm anything so they’re not bad, per se, but they’re definitely not particularly impactful when to comes to security so nobody else really thinks it’s worth implementing them. There’s a lot of stuff like this in OpenBSD, but to be fair a lot of other projects are also bad at making mitigations that aren’t very useful. Reading writeups on CTFtime or, heaven forbid, imagining what exploits would look like is sadly too common.