You're not sure it's allowed on YouTube? It has to be, I see larger youtube channels doing giveaways all the time asking for you to sub to multiple channels.
Just because larger channels are doing something, that doesn't mean that it's allowed. Until recently, lots of big channels were making weird kids videos and now all those channels have been terminated. Likewise, a few years ago, lots of very big channels had tags in their descriptions. People thought it must be ok. Then YouTube terminated them for it.
With the new monetization rules, you can be sure that Youtube will be looking very very closely at anyone trying to cheat the system, particularly in terms of sub counts and view time.
YouTubers should base their decisions on the rules and not on what other channels are doing. When a channel gets a strike, you're on your own. It's no good saying "Well other channels are doing it"
Like I said above, I wasn't sure of the exact ruling but a quick 10 second search and I found 2 relevant sections:
"You and any third party may not manipulate metrics on the YouTube service, including numbers of views, likes, dislikes, or subscribers, such that those metrics fail to reflect genuine user engagement with the YouTube service."
Source: This comes directly from the rules for holding a contest -
https://support.google.com/youtube/answer/1620498?hl=en
and
"content that solely exists to incentivize viewers for engagement (views, likes, comments, etc) is prohibited." (I would assume the "etc" also includes asking for subs) - Source:
https://support.google.com/youtube/answer/2801973?hl=en