Of cause they support. We use T300 APs for public events all the time, and we see quit a lot such clients connected. I would say this is a bit interesting question -- all 802.11ac devices are backword compatible. It means that 802.11n clients can connect to this AP using modulations, as well as 802.11ac clients in lower signal conditions also step back to 802.11n modulations. Actually most of the time most clients work exactly in 802.11n mode, as currently most clients are 802.11n. Usually only some lucky clients with new radio and really close location fully benefit from new standard.
Eizens is correct. It's not even very feasible to "accidentally" lock out 11n clients with an optimization setting, as 11ac data rates and 256QAM tend to only be useful with relatively strong signal strength, so even 11ac capable clients might find themselves regularly using 11n rates.