The spelling is trying to visually transcribe a pronunciation that is sometimes cited as a characteristic of the younger speech with "loosened" articulation (incidentally, it was a hot topic on Twitter weeks ago).
Note that in this case, however, as an introspection of the "younger generation", what あいかあらず stands for is probably not literally like [[aikaːɾazɯ]] as it suggests, but [[aikaɰaɾazɯ]], which retains an approximant without the labial narrowing (in the prescriptive [[aikaβaɾazɯ]]). It is generally distinguishable among speakers who use this pronunciation from the strictly consonant-less variant, which is not unheard either in a casual speech.
Also, I wonder if は in 二十センチは is related to this kind of は?
Yes, but the answers to the post don't seem right. You should instead see this one: Does は mean 'at least' in this sentence?