Hello,
For me personally archiving isn't checked by default. (global plans)
i haven't checked source but i assume it's checking the archiving database settings in the Exchange service settings.
where mine is left default (empty).
For custom plans per org i think it will depend on hosting plan settings.
Regards,
Marco

Thank you for your answer so far!
I've checked hosting plan settings and exchange service setting and unfortunately  i could not find anything related to archiving that i could mess with.

For example for "recoverable items storage" i was able to override the default value to unlimited by hardcoding "-1" in the correspondent .aspx file.

You probably want to undo the hardcoded config.
If you change mailbox plans for your customers currently it will not apply in Exchange due to -1 value.
Exchange requires 6144 as minimum, save value is generally 6500 and above but it has to be an actual value, 0 or -1 is not recognized or valid in Exchange.

In regard to the archiving for me it's most likely due to Archiving database left blank which is something you can try (just to make sure thats the reason why for me it's by default unchecked).

n regard to the archiving for me it’s most likely due to Archiving database left blank which is something you can try (just to make sure thats the reason why for me it’s by default unchecked).

Can you please point me towards this option? I cant find it.