In the sentence 'Opportunity is missed by most people', there is no linking verb. It is an example of a passive sentence. The active form of it is, 'Most people miss opportunity'. The verb is 'miss' and its passive equivalent is formed by the auxiliary is + past participle of 'miss'.
"A copular verb (also called linking verb) is a special kind of
verb used to join an adjective or noun complement to a subject.
Common examples are: be (is, am, are, was, were), appear, seem, look,
sound, smell, taste, feel, become and get".
All participle forms (present or past participle) can act as adjectives when they modify nouns.
In the sentence 'A letter was sent', the verb is not 'was'
alone. It is the verb phrase 'was sent' (the passive voice formed
using the auxiliary was + Past participle of 'send').
Example: You prepared 10 letters. You sent 5 letters or Five letters were sent. You have five letters remaining.
Now you can classify as: Some are sent letters and others are remaining letters.
The verb 'are' here is a linking verb and 'sent' and 'remaining' are adjectives - the former past participle and the latter present participle.
Direct or indirect object can also be changed to subject of the
I gave you a letter. (Active Voice, you = I O, a letter =D O)
You were given a letter. -Passive.
A letter was given to you. -Passive.