There's decent reasons behind most of those, or at least consistent ones, a lot is around slimness which drives strength and material decisions.
> removable batteries
Those usually require relatively flimsy backs that slide fit onto the phone. This has problems with water proofing and it's a part that can wear and eventually fit poorly or fall off. It's a lot easier to waterproof/resist a phone when you can seal the whole back of the phone off from the outside world. It also allows using soft polymer batteries that can eek out a bit more volume without the protective bits removable batteries need to be safe(r) to handle.
On the more cynical side maybe they figured it'd induce people to replace their phones more often though I don't have data showing how much of a factor bad battery is in replacing a phone versus other stuff like better performance or FOMO.
> IR blasters
Honestly don't think I had a phone with one of these since my Palm Centro in high school and college and I don't think I ever used the IR feature on there. The only real use I see is slow data transfers (better accomplished with Bluetooth now) and controlling TVs. The latter isn't a huge draw for most people I bet because people generally either have one remote (that more often can control everything if setup properly either through ARC and HDMI control signals or the cable box remote also sending the TV signals when needed) or many remotes and are willing to spend money on a nicer universal like a Harmony. There's probably still some out there today but it seems like an extra part that most people won't care about so the extra fractions of a cent + to include it gets wiped out to increase profits over a few million phones.
> SD cards
This one is maybe a little more cynical. There's some justification for reducing the number of intrusions in the waterproofing seals and increasing the internal volume available to squeeze more battery in. I think another reason is to induce people to buy the more expensive version of a phone. If you include SD card expansion storage it's harder to upsell people on the larger more expensive storage options because usually SD cards are cheaper than the equivalent sized storage in a phone
> headphone jacks
This one I think is a mixed bag. Including the jack sets some hard design limits on the shape of the edge of the phone and it's a pretty large hole to plug against water and other intrusions. I think Apple might have had more coldly calculated reasons for doing it with the close launch of their own AirPods. Some phone companies have similar offerings where you could see a calculated plan to increase profits.
I think we're maybe also just reaching the more mature phase of the smartphone design. There's a basic set of features most people care about and optimizing for that pushes companies in a pretty similar direction. There's some margin for small experimentation around the edges but those induce compromises in the core features people judge on so that limits the market and profit of making those choices.
There's technology like foldable screens that might shake things up but they're pretty new with some big compromises at the moment so they haven't really hit the core incentives and forces that are corralling phones into the same tight design and feature list.