2sable, 6 5 bytes

6 byte version:

AIl-g_

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Explanation:

A        Push alphabet
 Il      Push lowercase input
   -     Remove all chars of input from alphabet
    g    Get length of the remainder
     _   Print negative bool, where length < 1 = 1 (true), length > 0 = 0 (false)

5 byte version, inspired by carusocomputing's 05AB1E answer:

lÙ{Aå

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Explanation:

l        Push lowercase input
 Ù{      Push sorted uniquified input
   A     Push alphabet
    å    Is alphabet in sorted, uniquified input?

2sable, 6 5 bytes

6 byte version:

AIl-g_

Try it online!

Explanation:

A        Push alphabet
 Il      Push lowercase input
   -     Remove all chars of input from alphabet
    g    Get length of the remainder
     _   Print negative bool, where length < 1 = 1 (true), length > 0 = 0 (false)

5 byte version, inspired by carusocomputing's 05AB1E answer:

lÙ{Aå

Try it online!

Explanation:

l        Push lowercase input
 Ù{      Push sorted uniquified input
   A     Push alphabet
    å    Is alphabet in sorted, uniquified input?

2sable, 6 5 bytes

6 byte version:

AIl-g_

Try it online!

Explanation:

A        Push alphabet
 Il      Push lowercase input
   -     Remove all chars of input from alphabet
    g    Get length of the remainder
     _   Print negative bool, where length < 1 = 1 (true), length > 0 = 0 (false)

5 byte version, inspired by carusocomputing's 05AB1E answer:

lÙ{Aå

Try it online!

Explanation:

l        Push lowercase input
 Ù{      Push sorted uniquified input
   A     Push alphabet
    å    Is alphabet in sorted, uniquified input?

2sable, 6 5 bytes

6 byte version:

AIl-g_

Try it online!

Explanation:

A        Push alphabet
 Il      Push lowercase input
   -     Remove all chars of input from alphabet
    g    Get length of the remainder
     _   Print negative bool, where length < 1 = 1 (true), length > 0 = 0 (false)

5 byte version, inspired by carusocomputing's 05AB1E answer:

lÙ{Aå

Try it online!

Explanation:

l        Push lowercase input
 Ù{      Push sorted uniquified input
   A     Push alphabet
    å    Is alphabet in sorted, uniquified input?

2sable, 6 5 bytes

6 byte version:

AIl-g_

Try it online!

Explanation:

A        Push alphabet
 Il      Push lowercase input
   -     Remove all chars of input from alphabet
    g    Get length of the remainder
     _   Print negative bool, where length < 1 = 1 (true), length > 0 = 0 (false)

5 byte version, inspired by carusocomputing's 05AB1E answer:

lÙ{Aå

Explanation:

l        Push lowercase input
 Ù{      Push sorted uniquified input
   A     Push alphabet
    å    Is lowercase alphabet in sorted, uniquified input?

2sable, 6 5 bytes

6 byte version:

AIl-g_

Try it online!

Explanation:

A        Push alphabet
 Il      Push lowercase input
   -     Remove all chars of input from alphabet
    g    Get length of the remainder
     _   Print negative bool, where length < 1 = 1 (true), length > 0 = 0 (false)

5 byte version, inspired by carusocomputing's 05AB1E answer:

lÙ{Aå

Try it online!

Explanation:

l        Push lowercase input
 Ù{      Push sorted uniquified input
   A     Push alphabet
    å    Is alphabet in sorted, uniquified input?