I read somewhere that one must put their shoes to the left if they take them off for prayer, is this obligatory or mandatory?
There's no ruling about that except the fact that in most cultures left has a negative association such as bad, dirty and sinful, while right has a positive association such as good, clean and correct.
In fact there are even ahadith allowing to pray while wearing shoes (of course there shouldn't be any najassa on them)!
Beside this there only cultural obligations. So basically if you enter a mosque you may find places or frames where you can put your shoes in or on. As between entering the mosque and joining a congregation you might change your position, but won't take your shoes with you it sounds rather useless to put them at your left!