treble ("double treble"). He was granted a testimonial match by Celtic in May 2018, played against a Republic of Ireland XI at Celtic Park. On 2 December 2018, Brown came off the bench as Celtic defeated Aberdeen in the 2018 Scottish League Cup Final to claim seven consecutive domestic trophies. At the end of the same month, he made his 500th appearance for the club. He had already achieved a personal milestone at the start of that season by playing in his 100th UEFA club fixture, becoming the first Scottish player to do so. 2019–2021 On 31 January 2019, Brown declined an offer to join new Australian team Western Melbourne FC and other approaches from abroad, instead extending his contract with Celtic until summer 2021, indicating that when it ended he would probably retire from playing. On 31 March 2019, Brown was involved in several incidents during an important Old Firm match against Rangers at Celtic Park, including a red card shown to Alfredo Morelos for elbowing Brown to the head, Ryan Kent shoving Brown to his head using a closed fist following Celtic's winning goal (missed by the officials, but Kent was later charged with misconduct by the league authorities) and being confronted angrily by Andy Halliday after the final whistle when Brown chose to celebrate enthusiastically in front of the small section of visiting supporters. Each time, Brown was seen smiling and laughing at the Rangers players involved, having successfully antagonised them. Responses to his behaviour varied while Police Scotland decided it was a matter for the football authorities. The SFA charged Brown with "failing to act in the best interest of Scottish football" for his actions at the final whistle, but at the hearing on 26 April, the verdict was not proven. Brown made his 600th appearance for Celtic on 6 December 2020, playing in a 1–1 draw with St Johnstone. He received a straight red card after coming on from the bench for striking Livingston player Jaze Kabia, in a 2–2 draw on 20 January 2021. On 25 March, it was confirmed that Brown would end his 14-year stay at the club at the end of the 2020–21 season, as he had signed a pre-contract agreement with Aberdeen. Aberdeen On 25 March 2021, it was announced that Brown would join Aberdeen on 1 July in a player-coach role on a two-year contract working as assistant manager