You can add things to the ducking dictionary.

"Look mister, I use a LOT of curses and it is a VARIETY. I don't have time to be manually adding EVERY swear that I use."

Honestly, I'm just replying for the algorithm because it's an interesting question and a lot of people sort by active or hot. Good luck!

This is set to change in the next iOS version. (In developer beta now; to be released in early September, typically, when the next iPhones drop). So, if you’re otherwise happy with the keyboard, I’d say just use Text Replacement rules (Settings → General → Keyboard → Text Replacement) for a few months.

I added some swear words as contact names, this is one way to get around the fucking censorship

I just added the curses to the normal text replacement settings in IOS and I have never had a ducking problem

I've been using the SwiftKey keyboard for a while and it allows any fucking word I want to use. It's really good at learning and predicting phrases I use often, including foul language, video game terms and words, etc.

Go to General > Keyboard > Text Replacement, and create an entry for each word you want, putting the same thing in the phrase field and the shortcut field. Then it won’t try to replace them every time.

I like Typefinity. (I have only used it on iPad, don't own an iPhone)

A no nonsense ios keyboard with a number row!

Privacy wise, they claim to not collect anything. It's not FOSS and also not free. One time $5. 100% worth it.

There's not too much visual customization, but a few themes that include dark, light and few colorful themes. For keyboard settings, there's everything you'd expect from a keyboard - not too much, but enough to disable things you might not want like click sounds and adjust autocorrect.

For the part you asked about; Playing around with swipe, it doesn't seem to work with curse words.

Honestly, thats probably a good thing. Its better for you and these keybaords' PR to lean on the side of caution (It could also be requirement from Apple - idk) You wouldn't want it to mistake "the duck sits on the birch log" for "the fuck shits on the bitch log".

Manually typing out bad words has no issues with autocorrection set to max. I normally leave it on "suggestions only" and just use the autofill suggestions as I type big words. Overall the swipe works, I don't normally use it, but I was able to "swipe" a few simple sentences. I can't give you a definitivite answer to "is it good", I type everything like a caveman.