You're right that it could be past simple but it could also be future: [that were to be] held today/in the future - [unreal future in this case] The festival, [that is] held every spring, ... - present habitual The event, [to be] held next Tuesday, ... - future simple It's a kind of participle clause. Or a reduced relative clause: Your 4th and 5th meetings, [which were to be] held today ...

Even if they just said "your meetings today" have been cancelled, you could argue "but they aren't happening today, so why call them meetings today?" You see, it's really not a big deal. It doesn't say "your meetings that were held today" - that would be really saying they happened. Simply saying "your meetings held today" is so generic it's acceptable - obviously they mean "your meetings that would be held today" / "your meetings that are due to be held today" but they are using a quick & lazy way of saying it, and most native speakers won't think twice about it, because it's clear what they mean based on saying "will be cancelled" which clarifies they didn't happen yet.

Past participles, when used as adjectives, do not refer to past time. For example, if I say "That song is played in the key of C minor" it does not mean that it was played that way in the past. It means it is always played that way. The reason "held" confuses you is that you are thinking of it as a verb. It is not a verb in "held today". It is an adjective.