[EDIT] Reason to advise the Merlin version is that it contains all the recent updates and a special wireless driver which is known to help most users who suffer problems with newer firmware. The old .270 (and older) firmware versions do contain various security issues.
After updating your firmware to the latest available (374.42 RMerlin version), I would also do the following:
Full reset to defaults via the gui and after it completes; do a manual power cycle (unplug the power cord, wait for a couple of minutes and then plug it back in).
Now, manually setup the router again for your network and ISP. Do not use a config restore file to do so (this will negate the reset to defaults).
Use a new ssid for your wifi bands; or on each device you've connected to your existing network; 'forget this network' and then re-associate the device to the router again.
With the above steps complete, you will have a 2014 level router that is as fast or faster than it ever was with the best security possible at this time.
Do note though that if you're using Guest Networks that does seem to be partially broken (the guest networks work, but the limit access to the lan isn't working as it should be).
Offering firmware .270 or any old version is like offering someone to remove the locks from their home.
I guess there was a good reason for Asus to remove it from the download site.
It is not a matter if, but when known vulnerabilities are exploited.
I know that the majority of routers of all manufacturers run old firmware because most users are unaware of the risks or even unaware of the existence of firmware. On purpose reverting to old vulnerable firmware is a different story which shall not be encouraged.
With the RT-N66U as pure AP behind a router there is not much risk at the RT-N66U end.
Risks are where Internet NAT and DNS queries take place (the usual router functions).