I have been a casual runner for over 10 years now and have run 1/2 marathons and a full marathon previously. During this time I was lifting as well, but more in a bodybuilding format rather than powerlifting. I have taken a 6-month break from running twice a week consistently and want to start getting back into it to train for a 1/2 marathon. During my time off, I started getting into powerlifting and have made significant progress where my body no longer feels extremely fatigued after a session. Is it possible to continue to make incremental progress powerlifting while training for a 1/2 marathon or should I expect to plateau and aim to maintain my strength?
Yes, although In the fitness world especially, excess cardio will drain your calories and reduce muscle which is needed for powerlifting. On the other hand, lifting heavy weights and gaining weight will make your body slower or fatigue faster. That being said, you can absolutely do both! Your progress will just be slower in each activity, and you might not reach your absolute best in either, but being a competitive marathon runner or powerlifter probably isn't something you're considering anyway I'd guess. You can do both and will make a lot of progress just keep in mind your progress will be slower in both and eventually your progress in both if you keep doing this for 5+ years might slow down unless you focus on one or another
There is a documented interference effect of endurance aerobic and strength training, so if you'll train for both it's to be expected at least that your gain rate will slow at least above a certain level of proficiency - if you're a newbie you can gain in both for a while. In other words, the theory indicates that the more advanced you are, the more pronounced the effect is.
But sure, be prepared to see diminishing progress and maybe even plateauing, depending on your current level and on how hard will you be training. Here are some techniques to reduce the effect (basically separate the endurance and strength training sessions as far as you can possibly manage).
Note that there are sports where high performance players actually do train for both endurance and strength, like soccer, for example. So it's not that you can't do it, or that you'll necessarily hurt yourself or lose all your gains. It all depends on your actual goals and you must set your expectations right.