I can’t find any references to the 133 series on the redesigned routerboard.com site.
Is 2.18 still the current version, and if not, where do I find the latest?
i think You are talking about board firmware. This is still 2.18 for older boards (133, 532, 333)
RouterOS image contains the firmware too, but for older boards no update.
(do You know what changing in firmware versions?)