Haskell, 60 bytes

(init<>reverse).(scanl(pure.(>>= \u->u:[' '|' '<u]))<*>tail)

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Explanation / Ungolfed

For a string (eg. "abc") we apply first

scanl (pure . (>>= \u->u:[' '|' '<u])) <*> tail

which is the same as

\str -> scanl (const . (>>= \u->u:[' '|' '<u])) str (tail str)

This repeatedly applies (>>= \u->u:[' '|' '<u])) (appends a space to each character in the range [33..]) to the str until there are that many strings as str has characters: ["abc","a b c ", "a b c "]

Now we only need to concatenate the result (minus the last element) with its reversed counter part:

init<>reverse

Haskell, (削除) 60 (削除ここまで) 59 bytes

(init<>reverse).(scanl(?)<*>tail)
a?_=do u<-a;u:[' '|' '<u]

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Explanation

For a string (eg. "abc") we apply first

scanl (?) <*> tail

which is the same as

\str -> scanl (?) str (tail str)

This repeatedly applies (?) (appends a space to each character in the range [33..]) to the str until there are that many strings as str has characters: ["abc","a b c ", "a b c "]

Now we only need to concatenate the result (minus the last element) with its reversed counter part:

init<>reverse

Haskell, 60 bytes

(init<>reverse).(scanl(pure.(>>= \u->u:[' '|' '<u]))<*>tail)

Try it online!

Explanation / Ungolfed

For a string (eg. "abc") we apply first

scanl (pure . (>>= \u->u:[' '|' '<u])) <*> tail

which is the same as

\str -> scanl (const . (>>= \u->u:[' '|' '<u])) str (tail str)

This repeatedly applies (>>= \u->u:[' '|' '<u])) (appends a space to each character in the range [33..]) to the str until there are that many strings as str has characters: ["abc","a b c ", "a b c "]

Now we only need to concatenate the result (minus the first element) with its reversed counter part:

init<>reverse

Haskell, 60 bytes

(init<>reverse).(scanl(pure.(>>= \u->u:[' '|' '<u]))<*>tail)

Try it online!

Explanation / Ungolfed

For a string (eg. "abc") we apply first

scanl (pure . (>>= \u->u:[' '|' '<u])) <*> tail

which is the same as

\str -> scanl (const . (>>= \u->u:[' '|' '<u])) str (tail str)

This repeatedly applies (>>= \u->u:[' '|' '<u])) (appends a space to each character in the range [33..]) to the str until there are that many strings as str has characters: ["abc","a b c ", "a b c "]

Now we only need to concatenate the result (minus the last element) with its reversed counter part:

init<>reverse