What do examiners consider to be "memorised language", and how will your score be affected if you use it?
Individual words will not be considered memorised. However, if you have learnt some "big" words, you should make sure that you can use them in a natural and appropriate way, because incorrect use of words will affect your vocabulary score.
It's fine to memorise and use "topic vocabulary" phrases (e.g. the opportunity to engage face-to-face) and short organising phrases (e.g. people have different views about...). However, I recommend that you avoid learning long "any essay" phrases (e.g. ... is a controversial and hotly debated issue in today's world). These "phrases for any essay" are very obvious to the examiner, and you'll probably get a lower score if you use them.
Again, "topic vocabulary" is fine, but "any essay" sentences are not. So, if the question is about environmental problems, you can use a full sentence from my blog (e.g. Human activity is a major factor in the rise of the greenhouse gases that cause global warming). Having said that, it isn't easy to memorise full sentences for a wide range of topics, so I advise people to learn shorter "topic" phrases instead.
Paragraphs and full essays
If you write a perfect paragraph or full essay from memory and it fits the question, the examiner will not know and you will not be penalised. However, you would need to memorise hundreds of essays, so I don't recommend that you take this approach! Furthermore, if you write a perfect essay but it doesn't match the question, you will score zero!
My final tip
I recommend that you learn words and phrases (and maybe a few full sentences) for common IELTS topics, as well as some simple words and phrases for linking and organising your ideas. Don't learn lists of "phrases for any essay".