As an aside, BlueCoat is not a very reputable company. They are responsible for the government-sponsored censorship of Burma's and Syria's internet[1]. Which means that Symantec is currently the (American) company responsible for the censorship blacklist of Syria and Burma.
Two points here, about both the advice and the people giving it.
Regarding the advice, personally I think the advice is bogus. A lot of Mastodon instances have started legitimately using unconventional newTLDs. And I seem to see more URI shorteners, .com and .ru in spam than all the newTLDs put together (zero, from a hacked site, costs less than free). Country K-lining, while attractive to the lazy network operator, only works as an extreme temporary measure in a crisis - spammers adapt, but blocklists tend to only grow. And perhaps Symantec, given their business dealings with Verisign, might not be a 100% neutral party in making recommendations seemingly targeted primarily at severely disrupting the present and future business of cheaply-available TLDs?
Regarding Blue Coat, research shows Blue Coat devices are also used in the censorship/mass surveillance programmes of: Russia, UAE, Bahrain, Iran, and even China. Please also remember Blue Coat devices intercept, log and parse near-everything that goes through them. That puts them at a significantly elevated security risk above a network which didn't have them at all. I know I would find it unethical to report any vulnerabilities to that vendor, and I know I am not the only one who thinks so. And middleboxes like that are incredibly frustrating to the interoperability of the internet and present probably the single biggest hurdle to progress in internet protocols - ask someone in the IETF TLS Working Group currently working on TLS 1.3 just exactly what they think of them!
The federated structure of Mastodon means that, so long as I'm accessing toots via my host instances, the source of the toots doesn't matter. That plumbing is managed by the instances, not my local network gateway.
(If I were locally hosting, the situation would be different.)
Punching holes as needed would be another alternative.
I'm aware of the various arguments in favour, and opposed to, various forms of security blocking or not. I've participated in those discussions for most of the past 30 years. There are times when the onslaught simply becomes sufficiently excessive that measures need to be taken.
DNS namespace is large. I'm not going to independently add every last damned host, or domain, by hand. And even with blocklist subscriptions, the overhead is substantial.
I suspect this is a situation which may come to a head in the not-too-distant future, though timing such matters is difficult. The consolidation of much Web activity to a relatively small number of sites already reflects this in part.
[1]: http://surveillance.rsf.org/en/blue-coat-2/