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Idea is about using encrypted pastebin/clipboard/cupboard (i don't know, which name should I use, but probably clipboard is the best).

Problem is other person can read clipboard content from our computer, where we are leave computer. Screen looking is no solution, because:
1) Other program can read our clipboard content
2) We can forgot to look screen
3) Etc.

Clipboard encryption process will start:
1) when we don't use it for a long time
2) When Plasma5 detect we are leave our PC (by bluetooth for example)
3) Possible others
4) When we perform screen looking
And user could order to encrypt clipboard by themselves.

Clipboard content will be encrypted by public key and private key (to decrypt content) will be saved in encrypted form, so user must provide password to decrypt clipboard. You may notice, that clipboard content can be encrypted multiple times (not the same content, but newer content), but user will decrypt entire content in one-shot.

Can you please clarify how "other people" can read your clipboard entries without having access to your open desktop session?
An unlocked desktop session certainly is a security thread, sure, but then the solution is to lock that, not only the clipboard. Locking the session is already possible in various ways.

Look for example at Spectre/Meltown.
I think, that if your password is last entry in Klipper, then any program can read it as from X clipboard. But sure - I don't know if there's any API to read from Klipper indirectly. I also don't know much about debugger programming, but in many idea exist option to attach to process.

If your reference to Spectre and Meltdown is meant to indicate that there _might_ be possibilities to read data from some running computer system bypassing the security measurements...
... then sure, that always is a potential thread.

The question is, however, what to do about that. Certainly the best idea is to implement and deploy fixes to the actual thread, so to handle the cause, not the symptom. Certainly one can try to work around an issue by trying to secure some arbitrary detail (like the clipboard). However this is questionable: it adds much complexity (which typically raises vulnerability), does not really make things safer (wouldn't such general threads like those you refer to also compromise the stored secrets used for encryption and decryption?) and makes the desktop even more difficult to use in daily life (keep in mind the "keep it simple" principle).

So I personally think this is a wrong way to think.
But that certainly is only my 2 cents...