For most recent ones you should get an alert for most. The functionality is part of the software so improvements comes as releases mature and addons are improved/installed. From memory, I think this improvement came later down the line after the immense migration done by the admins, which is why early warnings may not have had an alert sent. So this is what the option looks like our side:
The second check box only appears when you tick the first. Start a conversation is 'Send a PM', and 'Send alert' just adds a '1' to your notifications. We don't send an alert in certain circumstances. These vary but common reasons might be:
i) backdated warning - something we missed a while ago but want to tag as the expiration will have ended
ii) when we feel the report or current behaviour doesn't fully substantiate a proper warning yet, but we want to be aware of something. In this instance, the warning becomes more of a 'note' on the account - but the functionality provided technically labels it as a 'warning'. This might be used if a new signup looks like a spambot. So, in the future a mod can look and already see there were suspicions of it being a spambot, and can act with reasonable assurance
iii) a note to highlight that a previous warning is being ignored - this makes having to go through users histories a bit easier if there are examples to hand for context. Sometimes these will be report generated, or sometimes just picked up as a mod reads an interesting thread. These are usually followed up, if necessary, with a warning with that checkbox selected. Of course, some expire and just remain there as orphans though as users self correct.
Its not mean to be a comprehensive list but it gives you an insight into how we try to keep things running without being overly dramatic or a constant presence in everyone's notifications. One of the reasons this was chosen by the admins was because of the huge visibility and accountability mods can be held to. Everything is audited, logged and visible to prevent any one person going rogue. Generally, if we can't reach a full consensus the least restrictive/intrusive measure is applied or referred up to admins, so they can cast their more experienced eye over the situation for conflict of interest, fresh eyes etc.