"Coldfusion Developer/trial edition" message. But it's not.

No need to reinstall cf. Hope I caught you before you did. See my point 2 here for perhaps an easy solution to that problem. I then share some other info that--while you didn't seek it--seems pertinent to raise for someone in your situation. 

1) Indeed, let's first make sure others (who may want to offer suggestions) notice that you refer here to cf10. So this is NOT about any sort of problem with CF license activation (with the online Adobe licensing service), as that started only with cf2021. 

(And I'll hold off for a moment on why it's dismaying to hear you're on a cf version that's not gotten updates/security fixes in nearly 10 years.)

2) If you have your cf serial number you can just go into the cf admin, click the "i" icon in the top right, and that page has a field for entering the serial number. The license should take effect immediately. (Since cf2021, that serial number is entered instead on the new "licensing and activation" page of the cf admin.) 

If somehow you don't know your cf serial number, it was stored in the license.properties in the cfusion/lib for all the years it was running ok. If you take file system backups, you could find it there.

3) Sadly, in all those years there have been some pretty gnarly vulnerabilities discovered in various cf versions. cf2025, 2023, and 2021 are still updated by Adobe with fixes (cf2021 gets its last ones in 2 months, 5 years after it came out.) And since cf10 came out in 2012, it got its last updates in 2017--now nearly 10 years ago.

Indeed, it's entirely possible that this removals of the license key was perpetrated by an outside (or inside) actor, who may have leveraged a vuln that allowed them write access to that file. If you add it back only to find it removed again soon, this seems a likely scenario.

4) And while it's a bit like rearranging deck chairs in the titanic, note that you could at least follow the guidelines for the cf lockdown guide, the version of which is still available online.

Just beware that "closing the castle doors" does not address the bad guys already in the tower, communicating with and perhaps shuttling items with folks across the moat. That also means that if you simply move to a new server (or switch to using Lucee--or even Python or php), you need to address any bad guy files that got in those last (nearly) 10 years that your cf10 was vulnerable.

5) I appreciate such news is daunting, and leads some to freeze with fear. There are folks (myself or other consultants) who can help you rectify things--either as they stand on that server with cf10 (better locking it down, and/or removing bad guys), or in getting you safely to a new server. And of course we can help with the better solution of getting you on a supported Cf version, or to another cfml engine (both of which come with a price of dealing with migration/compatibility).

6) One way to look at things is that (while playing Russian roulette) you've avoided thousands in cf licensing costs: you'd have paid a total of about us$6250 if you'd have updated CF Standard each time to cf11, 2016, 2018, 2021, and 2023, getting a 50% upgrade discount on the approx 2499 cost each time--thus 5*1250 or 6250 ). Or you could have just paid the full price of 2499 for cf2023.

But now that Cf2025 had come out and is subscription only, someone on cf standard pays just a little more per year: us$760, as documented in the Adobe site in various places.  My point is that the cost of paying now for help is just repaying technical debt you've been accruing/avoiding. And this is to help either to stay on a better- (though never fully) secured cf10, and/or move (carefully) to a new server if the current one is comprised, or move to a new Cf version, or move to a different cfml server (Lucee, boxlang).

But again you just want to get cf10 not being the dev edition. I hope my point 2 above does that for you. Please let us know either way.