From what I see is the mainstream market rarely changes too much on this front. Tech market rates in general seem to be going up so that impacts everyone but the gap will likely continue.
If you look at the details of the roles they tends still to have a large emphasis on scripted testing in the form of test cases with a significant bias towards verification type testing.
When testing has this focus within a company they have perhaps accidently actively diminished the potential a tester can add so you tend to see trends script focused tester, automated tester > developer in terms of the way the see value and pay accordingly.
Because that is still a predominately mainstream view, many of those paying for testing services will also follow that expectation and expect cheaper rates for testing, that is if they want testing at all given a growing premise that developers can do scripted testing very well.
Some testers may also propagate this view of testing particularly if the mainstream view is all they know.
There are though loads of testers even within this mainstream view that add much more value than this but its challenging for that value to get recognised because company mindsets are hard to change on this.
On the other side I saw a post recently that talked about outliers, it slips my mind who posted it but I identified with the thinking in that post. These are companies and individuals in my view seeing much more value in testers and what they can add to the teams with much more of a bias towards discovery rather than primarily verification. These outliers generally need to also be highly technical these days and offer broader value potentially with consultancy skills too. I feel like there is a trend for those who recognise this to pay at least on par with developers if not higher.
Those roles as outliers though are still not the mainstream and whilst a mainstream company might have 10 verification type testers at a lower rate the outliers will often add more value with just one or two discovery focused testers.
That makes these higher paid roles rarer. Now what if the mainstream moved to have much more of a discovery bias for their testing? I suspect average remuneration would increase significantly for testers but a lot of testers within the mainstream would not be required. Good roles increase but the market for testers in general may shrink five fold if that happened. That potential reality could hold back change from testers themselves.
Its a much bigger discussion on the industry as a whole and pace of change has been incredibly slow over the last 20 years or so.
Anyway just my take, note I am naturally speculating with quite a few thoughts above. Its a mixed bag of trends as usual.