The last cardinal deacon to be elected pope was Leo X (#218), the youngest son of the famed Lorenzo ‘Il Magnifico’ de’ Medici of Florence, in 1513 — he having been created a cardinal, in ‘secret’ (as opposed to in pectore), when he was thirteen. He is also reported to have famously said, when elected pope, ‘God has given us the papacy. Now let us enjoy it.‘ This, alack, was not to be. His papacy was buffeted, majorly, by Martin Luther’s Reformation. << Page 36 of my ‘The Next Pope‘ >>
That we haven’t had a cardinal deacon elected pope since 1513 should not be viewed in anyway as being ‘sinister’ or representative. Part of the reason being that many cardinal deacons had used the prevailing jus optionis preferment rules to become cardinal priests or cardinal bishops before they got elected as pope. Right now, one of my papabili, Italian Cardinal Agostino Vallini, the Vicar General of Rome, is a cardinal deacon.
There has never been an issue about electing cardinals deacons as pope.
A Roman Synod, convened in 769, Stephen III (IV) (#95), stated categorically that ONLY cardinal priests and cardinal deacons were eligible to be pope! Since then, this eligibility of cardinal deacons to be pope was never rescinded. So it has always been possible, as of 769, for a cardinal deacon to be elected pope.
Here is a table of eligibility (and papal electors), << Page 86 of my ‘The Next Pope‘ >>, that highlights the eligibility and role of cardinal deacons in papal elections — as of 769.
The reason is I did not go past 769 has to do with the above mentioned, pivotal synod. This synod was the first time that the term ‘cardinal’ was explicitly used in the context of papal elections. I am loathe to arbitrarily classify Roman clerics ‘cardinals’ prior to 769 because I worry that we might potentially run into an ‘apples’ vs. ‘oranges’ situation. Hence, the 769 cut-off. There are quite a few deacons that became pope prior to 769 — including Leo I ‘the Great’ (#45). The problem is that we can’t be sure as to whether all the deacons in Rome, prior to 769, should be considered cardinal deacons.
Since there have been 171 popes elected since 769, the 22 cardinal deacons elected pope represents 13%.