This is a common word, so I don't know why Taiwan's Ministry of Education omits this character,
Some dictionaries deliberately ignore characters for Cantonese.
In Cantonese, it is very common to borrow homophonous characters to represent another meaning, especially spoken words with forgotten source or words come from foreign. To prevent confusion, mark them with semantic component 口 to indicate them as spoken form.
- 咁, borrow and differential from 甘
- 嘅, borrow and differential from 既
- 嘢, borrow and differential from 野
- 吓, (一吓, 兩吓) differential from other meanings of 下 (now 下 only)
- 吋 = British inches (differentiate from Chinese 寸)
- 呎 = British feet (differentiate from Chinese 尺)
- 𠸄 = England/English (now written as 英 only)
- 𠸄咭唎 = 英吉利 = England/English (Britain/British)
- 嗬囒 = 荷蘭 = Holland (the Netherlands)