This option is here under the advanced settings -> click the link, then login with user root and password same as gl ui -> navigate to network -> wireless -> edit the wireless network -> wlan roaming
^ also see @bruce post if you scroll a bit above he has a screenshot to it.
however it is not inside the gl ui, that is normal 
Hmm i gonna be honest with you on this one because it is complicated and while gl-inet may choose to make op24 stable or beta at some point this is still more focused about their gl sdk than openwrt base.
In openwrt terms the 24 base is still picked from the main branch, this is highly snapshot although openwrts times for stable releases can often take about 6 months or a year, currently since about a month they finally got releases but i don't think the gl firmware is now based on these releases yet, i wonder if they plan to use these releases as base.
There are many times when a snapshot looks stable and there gl-inet branches off to build the sdk4 from, the stable wording is often referenced/focused to gl sdk and doesn't always follow the OpenWrt release system (though with that said this can be changed due to stable releases by OpenWrt this month).
So you have two things: OpenWrt and the GL sdk 4.x which is the custom scripting and ui around OpenWrt.
it keeps support for adblock and vpn.
If it is the same firmware?
This is hard to awnser, the gl sdk may has fixed or made subtile changes this can sometimes add new issues but also fix them the gl sdk4 is still in beta too if you read more about their plans, for openwrt this can also introduce new issues but in normal circumstances going higher on openwrts base is more stable ( i compile openwrt myself without gl source ;-), only issues i encountered at present are: some minor apk package manager issues, and dnsmasq.d problems )
One of the things in later OpenWrt firmware which are fixed:
- multicast / certain packet length crash on wifi is fixed (was a firmware bug in the calibration data)
- the mtk-soc time-out and a couple of mtk-soc crashes are addressed (though im not entirely sure for this one if it is fully fixed or that it still needs to be merged it surely have some commits merged for this for sure).