You can set a boundary without being rigid by pairing a quick expectations chat with consistent behavior. In your next 1:1 say, "I want to be responsive during business hours and also protect evenings, since my hours are 9–5." "Is it OK if I treat after-hours Slack pings as next-day unless you mark it urgent, and for true emergencies you call or use Slack's notify anyway?" Tie to reviews with, "I aim to respond within the same business day and by 10 a.m. next business day for anything that arrives after 5." When a late ping arrives and it is not a fire, reply in the morning with "Got it, handling now" or schedule that reply so you do not train people that you are available at night.
On Slack, set a notification schedule so you do not get buzzed at night. Desktop steps: click your profile photo, Preferences, Notifications, Notification schedule, set Mon–Fri 9:00 a.m. to 5:00 p.m., and mirror the same in the mobile app under Notifications. Turn on a status after 5 by clicking Set a status and writing "Offline for the evening, back at 9 a.m." with Clear after set to tomorrow morning, and add your phone number to your Slack profile for urgent calls. For emergencies, agree on a clear trigger such as production down or a customer escalation, and the path is call or text because DND will suppress Slack unless they choose notify anyway. For non-urgent asks, use Slack's Remind me about this tomorrow at 9 a.m. and Scheduled send on your reply. If you do need to acknowledge at night without working, say, "Saw this, will pick it up at 9 a.m. If you need it tonight, let me know and I will adjust."