The options added to the schedule command in 1.15 made it unintuitive. The first argument you need to provide is a choice between function and clear.

I suppose the function argument was originally added because this command was intended to schedule other things. If it is still relevant, it should also be used before clear.

Moreover, the append/replace argument was added as the last parameter. I also suppose this was done to prevent command breaking in 1.15. However, a lot of updates changed commands to make them more intuitive and respect some syntactic rules so the reasoning behind the changes of this command seems counterproductive.

The following syntax is an example fixing the two previous problems while keeping the function parameter:

schedule function (append|replace) <function> <time>
schedule function clear <function>

Original syntax (from the wiki):

schedule function <function> <time> [append|replace]
schedule clear <function>