Restricting user from entering text after certain character limit

No, you cannot prevent the user from continue typing.

In case this is a deal breaking requirement, which I have my doubts on, this would call for a custom component plugin.

You can try using the below code:

a!localVariables(
  local!description,
  a!textField(
    label: "Description",
    instructions: "You can only add 100 characters in this field, system will auto trim the characters after 100th character",
    value: local!description,
    saveInto: {
      a!save(local!description, left(save!value, 100))
    }
  )
)

This way it will never throw the validation but auto trim the characters after 100.

I have found it LARGELY sufficient to simply utilize Appian's newer, built-in "character limit" functionality with "show character limit" turned ON.

It does not physically STOP users from entering more characters than the limit, but it is VERY visible and makes it VERY easy for the user to comply.

If you MUST have something like you describe (which i'd be curious to hear the business owners' rationale behind), then a solution like HARSH posted above but with "refresh after keypress" turned on, will be your closest option.  It won't work perfectly though.  (You can use the character limit visibility to your advantage here too).

Edit: actually "refresh on keypress" doesn't help a whole lot here - either way, here's my revised code (shortened the limit temporarily for easier testing)

a!localVariables(
  local!description,
  local!limit: 20,
  
  a!textField(
    label: "Description",
    instructions: "You can only add " & local!limit & " characters in this field, system will auto trim the characters after 100th character",
    value: local!description,
    refreshAfter: "KEYPRESS",
    characterLimit: local!limit,
    showCharacterCount: true(),
    saveInto: {
      a!save(local!description, left(save!value, local!limit))
    }
  )
)