I do not believe ASN sharing is illegal or prohibited, it's not prohibited in LACNIC and in APNIC policy I also could not find anything about ASN sharing, only
APNIC policy states:
"2.3. Autonomous System (AS)
An Autonomous System (AS) is a connected group of one or more IP prefixes run by one or more network operators under a single and clearly defined routing policy.
2.3.1. Autonomous System Number (ASN)
An Autonomous System Number (ASN) is a unique two- or four-byte number associated with an AS. The ASN is used as an identifier to allow the AS to exchange dynamic routing information with other Autonomous Systems."
Nothing prohibiting ASN sharing and 2.3 specifically states "run by one or more network operators... single routing policy"
Regards
George
Message: 19
Date: Mon, 20 Mar 2023 16:24:09 -0400
From: ayang@august.tw
To: Collider <large.hadron.collider@gmx.com>
Cc: nanog@nanog.org
Subject: Re: Spamhaus flags any IP announced by our ASN as a criminal
network
Message-ID: <5b7ed1b1fbff65dfc63d188c2e1f95af@august.tw>
Content-Type: text/plain; charset=UTF-8; format=flowed
Several Huize ASNs, e.g. AS47158 and AS141011, were revoked due to RIR
policy violations, which include prohibited sharing of ASNs with third
parties, IP hijacking, and malicious path prepending.
Given this history, it is not surprising that Spamhaus would blacklist
IP addresses associated with their ASN. In my opinion, such action is
well-justified.
Best regards,
August Yang