I'm past turn 300 in a TA 1.96 game, as Thalans on an immense map that started with max rivals and generous worlds. We've lost one civ so far and a while ago I formed the first alliance in the game, with the Iconians.
More recently, the Iconians formed alliances with two other civs that are Close to me. But when I open the Diplo page for those civs, an Alliance is not available to offer. I can imagine there being reasons one of these civs would turn down my offer, but if we are Close and they have an existing alliance, shouldn't the option at least be visible in the UI?
On a possibly related note, this situation made me notice that no alliance formations are listed on the Events tab for this game. (I also am sad that the filter options are gone from this UI context and wish we could sort by column headings and use the Page Up and Page Down keys to scroll the list.)