There are a ton of dead end technologies that litter the landscape. Here is just two of my favorites...
The old acid warhead missiles from the ancient Field Manual for instance. You would think that with the prevalence of FF armor on so many units, someone might have continued to push these but they fell off the face of the earth. They were not even updated to the more modern rule sets. Guess having all the disadvantages just didn't balance out there.
The infamously fun Dead Fire missiles. Oh, how I wish these would have continued to be a thing instead of being left behind in favor of the MRM. Like the old AX missiles, these just were fun to play with and long before the ATM, hitting someone with a albeit garbage Dead Fire LRM-20 for potentially 30 to 40 points at long range was just hilarious or smacking someone upside the head with an 18 pt SRM-6 salvo was just nice. Sure, it means that you blow up extra-spectacularly on crits and are a walking bomb but heck, isn't that supposed to be half the fun? Oh well, they were deemed not to have continued... luckily I still have the old books and will still gladly throw trucks armed with DF-LRM-20s out there, they are going to blow up anyways, what is a little bit more pyrotechnics?
The Hyper Velocity Autocannon. Wow. Really. In theory, this was so much fun. In practice, ehhh... usually had a tank per company blow up per turn from self immolation. But the results were fun for as long as the luck held. Had they not put the self-destruct detail in on this weapon, would have worshiped the paper it was printed on. Real shame there too... always love the idea of a range 20 10 pt ballistic attack or a range 28 5 pt direct fire strike, rivalling the old AC-2 in actually being useful... then again, the HVAC-2 was spewing shots at range 35, often making one wonder if there was ever an open area large enough to actually be able to make use of this crazy almost artillery like range. Sadly, this feels ultimately like a failed/dead end development thanks to that whole explodes every other shot thing (4 AC-2s firing every turn from 4 tanks... you are going to get a 2 pretty fast.)
Too bad some of these ideas couldn't be resurrected elsewhere in different or more reliable forms. And yes, I know the DF missile became the MRM.