It’s a barrier, but it’s not detrimental. Grades are just one factor of many in an application. Sure, at some firms they may have more weight, but even then you can make up for it if the rest of your application is good.
I’d suggest avoiding LinkedIn profiles. The candidates with top grades are more likely to list them on their profile. A friend of mine got a third in his first year and starts at Linklaters in September, but he’s not going to be posting those grades on LinkedIn.