Purpose is to use single Master item for really polling data, which can be used for LLD and for polling data for discovered items.

For example on a video https://www.youtube.com/watch?v=XcztWztGjZA the expert uses a single Master item with update interval 5 seconds for both: perform LLD and as a Master item for discovered items.

The small update interval in 5 seconds (or for example 60 seconds) is fine for discovered items, but it's bad for LLD rule, as it's too often.

Of course it's possible to create one more Master really polling data item, to separate polling and discovering intervals. But that's not an elegant way .

What if single Master item could have small update interval, but based on it, we could create a dependent item with preprocessing step "Discard with heartbeat" and a value with, say, 1 hour?
And then use this dependent less-frequently-updated item as another Master for LLD rule?

It looks to me like a quite smart approach.
We just need the new "Discard with heartbeat" step.

I think it's easy to implement - copy an existing function for "Discard unchanged with heartbeat" and simplify it a bit.