Information about whether a character has prayed or completed a building is not and never has been stored - so they can't back-date the achievements.
I disagree with the above statement.
Prayer & construction records are kept for TWO weeks [just look at your reports].
Further as far as I know, construction HOURS and donations to town treasury are PERMANENTLY KEPT with two limitations.
#1. Info ONLY is in the cit(ies) where you did the work/made the donation.
#2. Info ONLY APPEARS IF you are a member of that city, ie if you have left the info doesn't show, but if you go back I am 95% sure the info regarding construction hours & town donations will still be there.
If I am wrong, I am sure someone will let us know, but I know whenever I rotated in/out of a town, it kept a running record of that info.
EDIT: Further I just looked at my OLD messages.
You know how in the past if the player left, instead of their name appearing in the "Conversational partner" column, there was a generic "System-Message" name. Well NOW the actual name is back, ie "Name123 (deleted)". And this is true EVEN FOR ANTIQUE messages, like from LAST DECEMBER. That tells me ALL THE INFO about a character is probably lurking around somewhere.
PS: I wish the (deleted) part was BEFORE the name, rather than AFTER the name, it would make it a LOT easier to see which people are still around & which are not.