My carry-on is 21.59 cm deep.
Will it fit the Ryanair sizer boxes? (which I am aware are just a little bigger than their specified size).
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The carry on luggage size is defined in Ryanair (https://www.ryanair.com/gb/en/useful-info/help-centre/faq-overview/Baggage#0-1). Yours does not comply, and in that sense you may be required to pay and send it as checked-in luggage.
Fitting or not in the airsizer boxes may be irrelevant because they can just pick a measuring tape and check it (I have seen this being done). Most of the times they also allow the luggage if you are able to push it into their measuring boxes. Even if the luggage is a bit larger, if somehow you can squeeze it inside than you are allowed to board (I have seen this too).
Lately, Ryanair became a less obsessed with this and for some time now I haven't seen inspections. It actually became nice to fly with them again. You will probably make it, but you should be aware they can measure it. It's up to you now.