Oh no, you need to find a line of text in a string that contains the word 'YES'. How incredibly difficult. Are you even a programmer? This is entirely trivial.
Maybe they’ll write « yeah » or « YE » or « oui », because that’s what users do
Those users absolutely can write 'YES'. They're perfectly capable of doing so. They don't do so because they're lazy. We all know people that simply refuse to read any kind of instruction on a computer screen. They will skip past any and all prompts. Guess what? That's their problem, not yours.
Stop enabling laziness!
If you can't create a way to say "yes", which is at least as easy to use as checking a checkbox and clicking a button, why should I assume you're capable of, well, much of anything.
It's not that it looks unprofessional. It looks either incompetent, or deliberately obtuse. In either case, I'll take my business elsewhere.
"Yes alright renew my subscription, just make it start in two weeks this time"
Your "solution" fails immediately here, because it gives people a false idea that they are in a dialogue with you whereas they are actually just expected to reply in a binary way.
Finding "a line of text in a string that contains the word 'YES'" is indeed trivial, however if that's all you can think of you should not be employed in any position of responsibility as it demonstrates a profound lack of judgement, understanding of how real people behave, and general business acumen.
The point of a business is to make money. You make more money by making the experience for your customers as easy as possible. If you don't, another company will.
Or worse then, like idiots that cannot understand a simple sentence and write back « YES »