I don't like the idea of a permanent banner, but a post in Important Announcements isn't a bad idea, especially since the FTX contagion is likely to cause several more bankruptcies over the coming weeks/months. Because the FTX issue is likely to trigger bankruptcies soon, this is time-sensitive, and so it'd be best to post it in the next ~36 hours.
A quick draft:
Reminder: do not keep your money in online accounts[/b]
When you log into a site and it says that you have a balance of "1 BTC" or "1 USD", this does not actually mean that you have that money. Rather, it means that the company showing you this balance owes you that money. In other words, you've given a loan to the exchange. Maybe the exchange will pay you back, but history is littered with defunct exchanges which were widely trusted at the time but ended up not paying back their depositors.
To the greatest extent possible, you should always avoid keeping BTC (or anything else) in online accounts. This is especially important right now because a very large exchange went bankrupt recently, and this may cause a contagion effect which will take down other sites which accept deposits.
Ideally, you should keep your BTC in a wallet where only you control the private keys. Here are some good options:
<insert list here>
Also, note that many DeFi systems are either fundamentally centralized despite using DeFi tokens/contracts and are therefore about as risky as online wallets, or they are potentially vulnerable to the FTX contagion due to the way that they're designed. <maybe insert more details here?>
Which are the best wallets for total newbies? Anyone who will actually need the advice in this post will need something very easy, so I don't want to list something too difficult. Probably I should just list one app for iPhone, one app available on Android via Google Play, and maybe one hardware wallet. (Linux/Windows/Mac wallets are too risky for newbies, I think.)
I'm not very familiar with DeFi systems. Should more be said about them?