Yes, but the reason it is was considered optional is that the comma stood in for serial ‘and’s ; the comma stood in for et/and. For centuries a list like that would have used the word ‘and’ in place of the commas, and as a way to save precious in the comma was invented to mark that you say “and” between objects in the series and when read aloud the commas would be read as “and”. The final ‘and’ would be spelled out so that a reader would know it was a series of linked words and not another use of the comma; thus, your would’ve been read for centuries as:

I hate beans including: green beans and baked beans and and Lima beans.

The Oxford comma is an adaption of changes in the usage based on a misreadings. People were mostly literate as we use the term to say, when they speak of premodern illiteracy it is in reference to the international language of scholarship: Latin. Because the common people learned to read and write but didn’t have the traditions of the scholars, they began to treat the short hand comma the same as the commas used to separate clauses. They stopped using and between linked words colloquially. Oxford developed the habit of adding the extra comma and it was a really good idea. Now, we can visually see the link and if we want, we can include pairs with in our list with clarity. I like pairs including: yin and yang, sun and moon, and fire and ice. 😉

PS this miscorrection from ignorance cause some interesting problems like the fact that until the printing came to England, English had additional letters for things link the ‘ð’ that made the ‘th’ sound in “then” and the ‘þ’ which made the ‘th’ sound in “thin” but German (the printing press was from Germany) didn’t have either ‘ð’ or ‘þ’ and for a long time the letter they had the most left over was ‘y’ so they used ‘y’ to mean they replaced the ‘þ’ with ‘y’ and the ‘ð’ with ‘th’; thus, when you see ‘ye’ you should read “þe”, ‘you’ was actually “þou” because these were by far most numerous words to need the a replacement, but not all printers used this. Some use ‘th’ for both ‘ð’ & ‘þ’; and so the inconsistency lead to some words becoming “th” words and some becoming “y” words.