There's a little metabolic down-regulation but nothing massive - the biggest thing that changes when dieting is NEAT/non-exercise activity often can go down the toilet as your body tries to sneakily conserve energy and reduce the size of the deficit - which is why it can be a good idea as well as tracking food intake to track steps etc since if you can force yourself to be consistent then you'll go some way towards countering that - people get lazy and move less. Generally with an aggressive deficit I'd be looking at
a) running it for a shorter span of time, like 4-6 weeks max with 10-14 day diet breaks in between if you're aiming for more than one cycle. It's unrealistic and unsustainable to diet hard AND for a long time
b) only going as low calorie-wise as you can without disrupting your ability to maintain intensity in the gym, since the aim of dieting is loss of fat whilst minimising loss of muscle tissue. There's only so low you can go before you're running on empty and start seeing big drops in performance, and muscle is metabolically expensive, so use it or lose it applies
I'm having some surgery done Monday and did a very aggressive mini-cut for 4 or so weeks at around 1700 calories. I made sure I walked 10-15k steps a day, plus lifting x4 a week and yoga x2 a week. Weight loss was pretty linear, I didn't alter calories once although I did drop some fat for more carbs, and my overall bodyweight dropped by 7.5% ish, although this will get knocked back a bit once glycogen is replenished. I only lost a rep here or there on a couple of exercises but nothing drastic and after the 4-6 week lay-off I'll hopefully have maintained my weight and look to gain for the rest of the year. Probably the leanest I've ever gotten actually.